An asynchronous transfer mode-passive optical network (ATM-PON) system is known as an example of an optical communications system.
The ATM-PON system is an optical communications system that has been internationally standardized by the ITU-T Recommendations G.983, and because it can realize point/multipoint transmission that enables a single optical line termination to communicate with a plurality of optical network units by a time division multiplex method, it is anticipated as a system that enables significant reductions in transmission costs.
With the ATM-PON system, because signals sent from the optical network units to the optical line termination are time division multiplexed and the optical network units and the optical line termination are connected at different distances according to subscribers, a receiver of the optical line termination unavoidably receives packet signals that vary greatly in signal intensity.
As with a general receiver, with the receiver of the ATM-PON system, an amplifier called a preamplifier (hereinafter, “preamp”) that increases the signal intensity of a received signal to a required level is incorporated at a prestage of the receiver.
Though a preamp has a predetermined dynamic range, its own gain must be controlled in some form in order to receive signals of a wide intensity distribution from weak signals to strong signals as in the ATM-PON system. Preamps of the ATM-PON system and other optical communications systems thus have gain switching circuits in many cases (see Patent Document 1, etc.).
Patent Document 1
Japanese Patent Application Laid-Open No. 2000-315923
Patent Document 2
Japanese Patent Application Laid-Open No. 2001-144552
Patent Document 3
Japanese Patent Application Laid-Open No. 2000-252774
The Patent Document 1 discloses a burst photoreceiving circuit that switches a feedback gain of a transimpedance amp instantaneously in accordance with a power level of an input signal.
The photoreceiving circuit performs control so that a conversion gain of the transimpedance amp is set to an intrinsic gain of the amp when an output level of the transimpedance amp is equal to or less than a reference voltage V1, the conversion gain of the transimpedance amp is lowered when the output level exceeds the reference voltage V1 and is equal to or less than a reference voltage V2 (V2>V1), and the conversion gain of the transimpedance amp is lowered further when the reference voltage V2 is exceeded.
However, because the photoreceiving circuit is arranged so that a switching element performs an ON operation whenever the output amplitude of the transimpedance amp exceeds a reference voltage, when the input signal waveform contains any of various waveform distortions, such as ringing, amplitude fluctuation, and signal sag, the gain switching is not necessarily performed at the head of the input signal and it cannot be predicted at which bit position in the input signal the gain switching will occur. The following of a threshold value was thus difficult.
Also, due to such waveform distortions, an erroneous conversion gain is set or an unintended conversion gain is set.
An object of the present invention is thus to provide a preamp gain switching circuit that can switch to an appropriate conversion gain according to the level of the input signal.